In our earlier studies of the effects of macromolecular crowding, we noted that the joining between DNA molecules catalyzed by T4 DNA ligase could be enormously stimulated by high concentrations of polymers. We have compared such crowding effects with those on the nonenzymatic cohesion between DNA molecules using assay systems based on the cohesive termini of Lambda DNA. High concentrations of several polymers greatly increase the rate of nonenzymatic cohesion, thereby allowing a test of the mechanism of the crowding effect on the DNA ligase reaction. The effects of macromolecular crowding were also examined with a different type of enzyme, T4 RNA ligase. Stimulation in the rates of joining of RNA substrates and the production of novel linear aggregates of RNA were obtained. Interactions with the Patent Office are proceeding in connection with the application on behalf of the Government of a patent covering the polymer-stimulated ligation of DNA.